1. Field of the Invention
The present invention relates generally to variable compression ratio internal combustion engines. More particularly, the invention relates to a method and system for diagnosing operation of a compression ratio setting apparatus of a variable compression ratio internal combustion engine.
2. Background Art
The xe2x80x9ccompression ratioxe2x80x9d of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). Generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. So-called xe2x80x9cvariable compression ratioxe2x80x9d internal combustion engines have been developed, for example, having higher compression ratios during low load conditions and lower compression ratios during high load conditions. Various techniques have been disclosed for varying compression ratio, including for example, using xe2x80x9csub-chambers and xe2x80x9csub-pistonsxe2x80x9d to vary the volume of a cylinder, see for example patents U.S. Pat. No. 4,246,873 and U.S. Pat. No. 4,286,552; varying the actual dimensions of all or a portion of a piston attached to a fixed length connecting rod, see U.S. Pat. No. 5,865,092; varying the actual length of the connecting rod itself, see U.S. Pat. Nos. 5,724,863 and 5,146,879; and using eccentric rings or bushings either at the lower xe2x80x9clargexe2x80x9d end of a connecting rod or the upper xe2x80x9csmallxe2x80x9d end of the connecting rod for varying the length of the connecting rod or height of the reciprocating piston, see U.S. Pat. No. 5,562,068, U.S. Pat. No. 5,960,750, U.S. Pat. No. 5,417,185 and Japanese Publication JP-03092552.
When controlling the operation of a variable compression ratio engine, it is essential to accurately and reliably determine whether a compression ratio setting device for varying an engine""s compression ratio is functioning as required. This is especially important, for example, when controlling ignition timing in the engine. In order to maximize fuel economy while minimizing engine knock, especially in high load conditions, it may be desirable to vary the engine""s ignition timing based on a detected compression ratio of the engine. See for example U.S. Pat. No. 4,834,031. In such case, a dedicated sensor, such as combustion pressure sensor or piston position sensor, is used to determine the compression ratio operating mode of the variable compression ratio internal combustion engine.
As such, the inventor herein has recognized the need for a method of diagnosing operation of a variable compression ratio device that does not rely on dedicated sensors within the internal combustion engine.
A method is provided for diagnosing operation of an internal combustion engine having a plurality of compression ratio operating states. The method includes the steps of determining a change in idle air flow of the engine as the engine is operated in selected ones of the compression ratio operating states; and evaluating operation of the internal combustion engine based at least in part on the change in idle air flow. Preferably, a first idle air flow is determined while the engine is operated a first of the compression ratio operating states, and a second idle air flow determined at a second of the compression ratio operating states. A difference between the first and second idle air flows is determined and an indication of engine operation provided based on a comparison of the difference to a predetermined limit. The indication can be for example a status flag set in computer memory, and/or an audible or visible indication. The disclosed method can be used with a variable compression ratio engine having discrete compression ratio operating modes, for example a xe2x80x9chighxe2x80x9d and a xe2x80x9clowxe2x80x9d compression ratio, or with a continuously variable compression ratio engine having a plurality of intermediate compression ratio operating modes.
Advantageously, the operation of a variable compression engine in one or more compression ratio operating states can be accurately and reliably determined without relying on one or more dedicated sensors, such as pressure sensors or proximity or position sensors. The methods described herein, which can be implemented solely in computer code, can be used for diagnostic purposes to evaluate operation of a compression ratio setting apparatus, such as a retractable connecting rod or piston head or combustion sub-chamber. The disclosed invention therefore reduces costs and complexity associated with the manufacture, assembly, operation and maintenance of a variable compression ratio internal combustion engine.
In accordance with a related aspect of the present invention, a corresponding system is provided for operating a variable compression ratio internal combustion engine. The system includes a sensor disposed within the engine for generating a signal representing idle air flow into the engine, a compression ratio setting apparatus for configuring the engine in selected compression ratio operating states of the engine, and a controller in communication with the sensor and the compression ratio apparatus for determining a change in the idle air flow of the engine as the engine is commanded by the controller to operate the engine in selected ones of the compression ratio operating states and for evaluating operation of the internal combustion engine based at least in part on the change in idle air flow. Preferably, the sensor is an air mass flow (MAF) sensor disposed within the throttle port of the engine. A notification device in communication with the controller provides information relating to the operation of the compression ratio setting apparatus to a vehicle operator.
Further advantages, objects and features of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.